Tsutomu Yoshitake

Site Identification of Carboxyl Groups on Graphene Edges with Pt Derivatives

Although chemical functionalization at carboxyl groups of nanocarbons has been vigorously investigated and the identities and quantities of the carboxyl groups have been well studied, the location of carboxyl groups had not previously been clarified. Here, we show that site identification of carboxyl groups is possible by using Pt-ammine complex as a stain. After Pt-ammine complexes were mixed with graphenes in ethanol, many Pt-ammine complex clusters with an average size of about 0.6 nm were found to exist at edges of graphene sheets, indicating that the carboxyl groups mainly existed at the graphene edges. These results will make it easier to add functionalities by chemical modifications for various applications of nanotubes and other nanocarbons.

Preferentially oriented epitaxial Y-Ba-Cu-O films prepared by the ion beam sputtering method

Preferentially oriented epitaxial Y‐Ba‐Cu‐O films were prepared on (100) SrTiO3 substrates by oxygen reactive ion beam sputtering. The epitaxial orientations were varied by controlling both substrate temperature and oxygen parital pressure. c‐axis oriented films tended to be formed at higher substrate temperatures (>620 °C) and lower oxygen pressures ( 3×10−3 Torr). The best Tc (end) of 82 K was observed in one of the c‐axis oriented film without post‐annealing. The tendency for preferential orientation can be well understood in terms of the lattice mismatch between the substrate and the film, the lattice constants of which depend on oxygen deficiency.

Epitaxial Y-Ba-Cu-O Films on Si with Intermediate Layer by RF Magnetron Sputtering

Epitaxial films of Y-Ba-Cu-O were obtained on Si substrate using epitaxial intermediate layer consisting of SrTiO3(or BaTiO3)/MgAl2O4. MgAl2O4 was epitaxially grown on Si(100) substrate by chemical vapor deposition, and then SrTiO3 or BaTiO3 was also epitaxially grown on MgAl2O4 layer by means of RF magnetron sputtering. Y-Ba-Cu-O films were prepared on SrTiO3(BaTiO3)/MgAl2O4/Si substrates by RF magnetron sputtering and their epitaxial growth was confirmed by RHEED observation and X-ray diffraction measurements. Epitaxial orientations of Y-Ba-Cu-O films varied in dependence on RF input power; lower RF power resulted in c-axis oriented film and higher RF power resulted in a-axis oriented film. Preparation of Y-Ba-Cu-O directly on MgAl2O4/Si was also studied, but only randomly oriented polycrystal film has been obtained so far. In sputter Auger depth measurement, any notable diffusion between Y-Ba-CuO film and the substrates was not observed. Resistive superconducting transitions with zero resistance at 65K on SrTiO3/MgAl2O4/Si and at 70K on BaTiO3/MgAl2O4/Si were observed.

A variable-emittance radiator based on a metal–insulator transition of (La,Sr)MnO3 thin films

Variable-emittance radiators based on the metal–insulator transition of (La,Sr)MnO3 thin films have been developed. The emittance property of the films was evaluated from infrared reflectance spectra; that is, the (La,Sr)MnO3 thin films show low emittance at low temperature but high emittance at high temperature. Moreover, the emittance property significantly changes at the metal–insulator transition temperature, where the material changes from a highly reflective (i.e., low emissive) metal to a less reflective (i.e., high emissive) insulator. The (La,Sr)MnO3 thin films fitted on a spacecraft surface can, therefore, be used to automatically control the emissive heat transfer from the spacecraft without the need for any electrical power. The developed radiators also greatly reduce the weight and production cost of the thermal control devices. The dependence of the emittance property on film thickness reveals that 1500-nm-thick films can be used for variable-emittance radiators.

Effect of oxygen deficiency on the crystal structure and superconducting properties of the Ba2YCu3Oy

Effect of oxygen deficiency on crystal structure and superconductivity of Ba2YCu3O7-δ was studied by annealing and quenching experiments. With increasing oxygen deficiency superconductivity was suppressed remarkably while axial ratio, a0/b0 in the orthorhombic phase approached to unity and c0 value increased. Structural transformation from orthorhombic to tetragonal lattice took place in the temperature range, 950~980°C and maximum oxygen deficiency, δ=0.7 was observed for the tetragonal phase.

Determination of the Orthorhombic-Tetragonal YBa2Cu3O7-δ Phase Boundary in the δ-T Diagram

The orthorhombic-tetragonal YBa2Cu3O7-δ phase boundary in the δ-T diagram was determined by a detailed isobar TG measurement under the oxygen partial pressures of 1 to 0.01 atm. The phase boundary was found not to be located at δ = const., but to have a slope of dδ/dT~-4.7×10-4 K-1. The experimental results are discussed with a simple order-disorder transformation model of oxygen atoms on the Cu plane sandwiched by Ba planes.

Epitaxial film growths of artificial (Bi-O)/(Sr-Ca-Cu-O) layered structures

Epitaxial film growth of artificial (Bi‐O)/(Sr‐Ca‐Cu‐O) layered structures on MgO substrates was carried out using reactive‐oxygen‐gas dual ion beam sputtering and shuttering technique. A 12 A Sr‐Ca‐Cu‐O buffer layer seems suitable for perfect epitaxial film growth. Single‐phase films having periodicities of 12, 15, and 18 A (which correspond to bulk 24, 30, and 36 A phases in Bi oxide superconductors) were selectively grown by adjusting the thickness of a Sr‐Ca‐Cu‐O layer sandwiched by Bi‐O bi‐planes. A high Tc phase film with a total thickness of about 300 A showed an onset Tc of 110 K and a zero resistivity temperature of 45 K without post‐deposition annealing. The shuttering technique seems to enable the layer‐by‐layer film growth, and it is very effective for the formation of smooth and perfect epitaxial structures.

In situ epitaxial growth of Bi2(Sr,Ca)3Cu2Ox films by ion beam sputtering with an atomic oxygen source

In situ epitaxial growth of Bi2(Sr,Ca)3Cu2Ox films was performed by ion beam sputtering in atomic oxygen ambience at the substrate temperature of 640 °C. The films showed an epitaxial growth in which the a and b axes were parallel to 〈100〉 MgO, and the superstructure according to the incommensurate modulation along the b axis was also observed. The superconducting properties of the as‐grown films seemed to sensitively depend on the oxidation treatment during the cooling down process. The zero resistivity temperature Tc(R=0) of a 600‐A‐thick film cooled down in the same atomic oxygen density as the film growth ambience was 60 K, but it increased up to 80 K after a post‐deposition annealing at 500 °C for 1 h in air. In contrast, as‐grown films cooled down in insufficient oxidation ambience showed the Tc(R=0) of 76 K without post‐deposition annealing. The control of the oxygen concentration is critical for the superconductivity of as‐grown films.

As‐grown superconducting Bi‐Sr‐Ca‐Cu‐O thin films by coevaporation

Superconducting Bi‐Sr‐Ca‐Cu‐O thin films have been prepared on (100) MgO substrates at about 600 °C by coevaporation. The c‐axis lattice constant of this system was controlled to the values of 24–43 A by changing film composition. Superconducting transition temperatures of these films were affected by substrate temperature and by a post‐deposition annealing at a low temperature. The highest zero resistance temperature (Tc, zero) of the as‐grown Bi2(Sr,Ca)3Cu2Ox film was 79 K. The best Bi2(Sr, Ca)4Cu3Ox film showed an onset temperature of 105 K and Tc, zero zero of 78 K after annealing at 400 °C for 1 h.

Highly efficient field emission from carbon nanotube-nanohorn hybrids prepared by chemical vapor deposition.

Electrically conductive carbon nanotubes (CNTs) with high aspect ratios emit electrons at low electric fields, thus applications to large-area field emission (FE) devices with CNT cathodes are attractive to save energy consumption. However, the poor dispersion and easy bundling properties of CNTs in solvents have hindered this progress. We have solved these problems by growing single-walled CNTs (SWNTs) on single-walled carbon nanohorn (SWNH) aggregates that have spherical forms with ca. 100-nm diameters. In the obtained SWNT-SWNH hybrids (NTNHs), the SWNTs diameters were 1-1.7 nm and the bundle diameters became almost uniform, that is, less than 10 nm, since the SWNTs were separated by SWNH aggregates. We also confirmed that a large-area FE device with NTNH cathodes made by screen printing was highly and homogeneously bright, suggesting the success of the hybrid strategy.